1. Field of the Invention
The present invention relates to a semiconductor laser element and a semiconductor laser, and more specifically to a semiconductor laser element and a semiconductor laser having an improved heat dissipation characteristic. The present invention also relates to a semiconductor laser element comprising a GaN-base semiconductor and to a semiconductor laser provided with such a semiconductor laser element.
2. Description of the Prior Art
Recently, the use of semiconductor lasers has been expanding remarkably. In many of these uses, there has been a demand for a semiconductor laser with higher output. Accordingly, for realizing higher output of a semiconductor laser, various attempts have been made to improve the structure of the semiconductor laser element itself. For example, reported in Literature 1), J. K. Wada et al, “6.1 W continuous wave front-facet power from Al-free active-region (λ=805 nm) diode lasers”, Applied Physics Letters, Vol. 72, No. 1 (1998) pp. 4–6, is a semiconductor laser having an active layer made of InGaAsP containing no Al, an optical wavguide layer made of Ingap, and a clad layer made of InAlGap, and oscillating in a 805 nm band.
Suggested in Literature 1) as the structure for lowering the photodensity of the active layer to improve the high output characteristic is an LOC (Large Optical Cavity) structure with widened thickness of the optical waveguide layer. The increase of the maximum light output thereby is reported.
Also known as the semiconductor laser having an active layer with no Al and oscillating in a 0.8 μm band is a semiconductor laser having, on an n-GaAs substrate, an n-AlGaAs clad layer, an i-InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-InGaP optical waveguide layer, a p-AlGaAs clad layer, and a p-GaAs cap layer, as shown in Literature 2); T Fukunaga et al. “Highly Reliable Operation of High-Power InGaAsP/InGaP/AlGaAs 0.8 μm Separate Confinement Hetrostructure Lasers”, Jpn. J. Appl. Phys. Vol. 34, (1995) pp. L1175–1177.
Also, to improve the heat dissipation effect of a semiconductor laser element, various structures of forcibly cooling the device with a cooling medium such as water have already been proposed. For example, proposed in Literature 3), Ray Beach et al., “Modular Michrochannel Cooled Heatsinks for High Average power Laser Diode”, IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 28. No. 4, April 1992, is a structure of a water-cooling mechanism for a semiconductor laser element using a microchannel.
On the other hand, about a semiconductor laser of a 400 nm band having a fine spot, required in the field of printing, etc., using an optical disk memory and a photosensitive material, is a beam having a high reliability and a high quality which oscillates in the optical density fundamental transverse mode with a Gauss distribution so as to fit for the increase of the image density and the increase of the image quality. For example, reported in Nakamura et al., Literature 4) InGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates with a Fundamental Transverse Mode, described in “Jpn. J. Appl. Phys. Lett., Vol. 37, pp. L1020, is a semiconductor laser obtained by depositing an n-GaN buffer layer, an n-InGaN clack-preventing layer, an n-AlGaN/GaN-modulated dope supper lattice clad layer, an n-GaN optical waveguide layer, an m-InGaN/InGaN multiple quantum well active layer, a p-AlGaN carrier block layer, a p-AlGaN/GaN modulated dope super lattice clad layer, and a p-GaN contact layer on a GaN substrate, the Gan substrate being obtained by forming a GaN layer on a sapphire substrate utilizing the selective growth using SiO2 as a mask and by releasing the GaN layer and a part of the sapphire substrate as the Gan substrate.
However, the structure shown in Literature 1) may lead to a phenomenon of COMD (Catastrophic optical mirror damage), where the end face of the device is damaged because of temperature increase of the end face triggered by an electric current generated by the light absorption at the end face, and because of smaller bandgap which increases light absorption at the end face. Therefore, the maximum light output is restrained to avoid the COMD. Since the light output which may trigger the COMD changes time-by-time, it sometimes happens that the operation of the semiconductor laser is suddenly stopped. As a result of such circumstances, it is difficult to obtain a high reliability at the time of a high output driving in the semiconductor laser proposed by Literature 1).
Also, in the semiconductor laser shown in Literature 2), the maximum light output is considerably low, in fact as low 1.8 W.
Furthermore, in the structure described in Literature 3) above, there is a problem that a large-scale cooling mechanism is required even in the case of cooling a single semiconductor device. Such a large scale cooling mechanism also requires a large place. Another problem is that it is difficult to obtain the sufficient cooling effect required for the recent high-output laser element because cooling is indirectly carried out from one surface connected to a module.
On the other hand, in the semiconductor device described in Literature 4), the reduction of the element resistance is attempted using a modulation dope super lattice clad layer but the reduction is not insufficient. Thus, the deterioration of the reliability due to the joule heat occurs during operation. Also, the resistance of the element is high in the semiconductor laser comprised of the semiconductor layers of the above-described system. Therefore, particularly in the laser of a single mode where the contact area with the contact layer is narrow, the influence of heat generation becomes a problem in practical use. The generation of the joule heat is coped with by cooling using a heatsink, etc. However, in the structure of the above-described semiconductor device, the heat dissipation is insufficient, because a heatsink is formed on the n-GaN layer exposed by etching the side surface of the semiconductor laser device making the form of the device complicated. That is, cooling from the p-electrode side near the active layer generating heat is difficult, and only cooling from the n-electrode side far from the active layer is possible. Also, because the p-electrode and the n-electrode are not disposed in a vertical direction but are disposed to the right and left, the stream of the electric current injected from the p-electrode is not straight but is liable to become inhomogeneous, whereby a uniform light emission where the optical density is a Gauss-type distribution cannot be obtained. For obtaining the light emission of a Gauss-type distribution, it is necessary to make the ridge width as narrow as possible. However, when the ridge width is narrowed, there is a problem that increase of the output is difficult.